Natural gas, a gas mixture formed primarily of methane which may also include ethane, propane, butane, pentane and higher molecular weight hydrocarbons, is a vital component of the world's supply of energy as a source of providing heat and electricity, and fuel for vehicles. It is also used as a chemical feedstock in the manufacture of plastics and other commercially important organic chemicals. Natural gas may be harvested or synthesized as a primary product or may be a byproduct of other oil exploration activities, and is abundant in the United States. Natural gas is clean burning and emits lower levels of potentially harmful byproducts into the air than some other fossil fuels. Natural gas is found in deep underground natural rock formations or associated with other hydrocarbon reservoirs in coal beds and as methane clathrates. Petroleum is also another resource found in proximity to and with natural gas.
Different types of natural gas are found in a variety of sources. Associated petroleum gas (APG), also known as flare gas, is natural gas found in association with deposits of petroleum. APG has been historically released as a waste product from the petroleum extraction industry. Due to the remote location of many oil fields, either at sea or on land, APG is considered a nuisance byproduct and is typically burned off using a gas flaring device. Over 150 billion cubic meters of AFG type natural gas are flared or vented annually by World Bank estimates, which is approximately equal to about 25% of the natural gas consumed in the USA in 2012. Shale gas is natural gas formed from being trapped within shale formations. Shale gas has become an increasingly important source of natural gas in the United States since 2000, when shale gas provided only 1% of U.S. natural gas production. With the development of hydrofracturing technology, by 2010 shale gas represented over 20% of U.S. natural gas production, and predictions indicate that shale gas will represent 46% of the U.S. natural gas supply by 2035. Coal bed methane (CBM) results when methane is adsorbed into the solid matrix of coal. CBM is also referred to as “sweet gas” due to its lack of hydrogen sulfide. CBM is distinct from typical sandstone or other conventional gas reservoirs, as the methane is stored within the coal by a process called adsorption. The methane is in a near-liquid state, lining the inside of pores within the coal. CBM typically comprises low levels of H2S and CO2. Biogas methane can be generated as a byproduct of anaerobic biochemical activity. Digesters, landfills and commercial biogas generators are used for converting man-made wastes into energy, i.e., biogas methane.
Natural gas is colorless, shapeless, and odorless in its pure form, and is combustible. It is one of the cleanest, safest, and most useful of all energy sources. When burned, natural gas gives off a great deal of energy and while producing few emissions. In other words, natural gas is clean burning and emits lower levels of potentially harmful byproducts than other fossil fuels. While natural gas is formed primarily of methane, it can also include ethane, propane, butane, pentane and higher molecular weight hydrocarbons. Natural gas does not typically exist as a pure hydrocarbon mixture, but includes other components as shown in Table 1 below. In some gas fields, higher molecular weight petroleum liquids can be associated with natural gas. These liquids bring additional commercial value to the natural gas.
TABLE 1Composition of Natural Gas - representativeMethaneCH470-90% Heavier hydrocarbonsC2H6+0-20% Carbon DioxideCO20-8%OxygenO20-0.2% NitrogenN20-5%Water vaporH2O0-1%ParticlestraceHydrogen sulfideH2S0-5%Rare gasesAr, He, Ne, Xetrace
Natural gas is generally classified based on the quantity of acidic gases present in the mixture, i.e., primarily hydrogen sulfide and carbon dioxide. Sweet gas is natural gas of a quality that is pure enough to be commercially used, typically <2% carbon dioxide and <25 parts per million hydrogen sulfide. In order to be useable, natural gas must either be sweet gas direct from the source or must be treated to sweet gas levels. Sour gas is natural gas or any other gas containing significant amounts of hydrogen sulfide (H2S), which can be as high as 25%. However, the threshold of what is considered significant varies by country, state, or even agency or application. Acid gas is generally classified as natural gas or any other gas mixture containing significant quantities of acidic gases, typically hydrogen sulfide (H2S), carbon dioxide (CO2), or similar acidic contaminants which can be greater than 25%.
A variety of environmental (problems are associated with harvesting natural gas. Generally, when a gas well is developed, i.e., completion, but before the natural gas is harvested for commercial use, the initial gas must be purged as it is generally not usable. Historically, the initial gas has been burned using flaring equipment. Regulatory agencies worldwide are putting substantial pressure to eliminate this practice and are moving to green completion strategies. Associated petroleum gas, i.e., APG, was historically not considered a commercial product. Generally, the main objective is to recover crude oil, and the associated gas is merely an unwanted byproduct that is just flared. As oil wells mature, APG can become increasingly contaminated with H2S and CO2. As such, flaring puts substantial amounts of carbon into the atmosphere. Moreover, flaring sour gas generates substantial amounts of SO2 which converts to H2SO4 in the atmosphere, which leads to acid rain. With more stringent regulations on flaring and greater public awareness of global warming, oil producers are faced with handling APG in a more environmentally way. Although the foregoing environmental issues are present regardless of the location of oil exploration, exploration and production activities offshore have limited options currently for handling APG, and specifically sour and acid gas. APG associated with ocean platforms raise serious issues as the exploration and production activities offshore presently have limited options for handling APG, and specifically sour and acid gas.
Known natural gas handling procedures suffer from a variety of drawbacks. Flaring has historically been the most common treatment method as it is simple and low monetary cost; however, it is likely to become increasingly restricted within the foreseeable future due to changes in various regulations. Deep well injection, i.e., reinjection of the sour gas, has also been used as a method of handling sour gas. It is believed that this activity merely delays the problem as the reinjected gas eventually may return to the source reservoir or well. With respect to conveying sour gas to shore from maritime platforms, various methods have been used to convey acid gas to shore. Since the gas is extremely corrosive, exotic and expensive materials must be used in building the necessary pipelines. Once the sour or acid gas arrives on shore, it still requires treatment to make it useable. Sour or acid gas handling may comprise amine process on platform or on shore. Before a raw natural gas containing hydrogen sulfide or carbon dioxide can be used, the raw gas must be treated to remove those impurities to acceptable levels, commonly accomplished by an amine gas treating process. The removed H2S is most often subsequently converted to by-product elemental sulfur in a Claus processor, or it can be treated in a wet sulfuric acid process unit where the by-product is sulfuric acid. The foregoing amine process has commercial limitations on high H2S and CO2 content streams. Furthermore, amine systems have limited commercial use on platforms. They are very expensive, large, require the use of chemicals and generate substantial amounts of process byproducts that need to be transported to shore for disposal or reuse.
Heretofore, the oil and gas exploration industry, has been faced with capturing and handling natural gas in an environmentally responsible way; however, known systems are complex and expensive. As sour and acid gas must be treated not only at land operations but also on maritime platforms, a solution is needed that can address a variety of problems. There has been a long felt need and thus the present method and system includes: a small foot print; a minimum weight; a simple, easy operating procedure; flexibility to treat different stream chemistries, such as it must be able to handle high concentration of H2S and down hole chemistry additions for controlling scaling and corrosion; use of minimum consumables and chemicals; minimum waste generation; cost effective operation; and, conversion of wastes to usable products; the capacity to substantially reduce the amounts of H2S, CO2 and water so that standard pipeline material can be used to convey natural gas to shore. Furthermore, there has been a long felt need for a method and system of producing fuel gas by treating acid and sour gas to an acceptable quality to be used to drive compressors and electrical generators on the maritime platforms. Still yet further, there is a long felt need for natural gas being of sufficient quality for use in enhanced oil recovery (EOR) so that it does not contaminate crude oil reservoirs as the gas is injected to recover oil. In view of the foregoing, it can be seen a novel system and method are needed to extract sweetened gas from sour and acids gas feeds that minimize the addition of carbon and sulfur dioxide to the atmosphere, while meeting water and air regulatory requirements.